In the papermaking art, an aqueous suspension containing cellulosic fibres, fillers and additives, referred to as a stock, is fed into a headbox which ejects the stock onto a forming wire through a slice opening. Water is drained from the stock through the forming wire so that a wet web of paper is formed on the wire, and the web is further dewatered and dried in the drying section of the paper machine. Retention agents are usually introduced into the stock in order to increase adsorption of fine particles, e.g. fine fibres and fillers, onto the cellulosic fibres so that they are retained with the fibres on the wire. A wide variety of retention agents are known in the art, examples of which include linear, branched and cress-linked organic polymers of anionic, non-ionic, amphoteric and cationic nature, organic polymers of different molecular weights, inorganic materials, and many combinations thereof. Due to incomplete retention, the water obtained by dewatering the stock and the wet web, referred to as white water or back water, contains fine particles not being retained on the wire and this water is usually recirculated in different flow circuits.
Due to non-uniform shrinkage of the paper web in the drying process, the resultant dried web usually has a non-uniform basis weight profile in a cross-machine direction. Notably, the shrinkage in the middle area of the paper web is lower than in the lateral areas, thereby producing a higher dry basis weight in both of the lateral areas of the web. In the past, a lip defining the slice opening has been controlled along its length to control the basis weight profile of the web. However, in practice, it is very difficult to obtain a uniform basis weight across the width of the web using this type of control arrangement. In addition, attempts to control the basis weight profile in this manner affect the fibre orientation profile of the paper web which, usually, results in adverse effects on the quality of the paper produced, such as anisotropy of strength and stretch.
Improved basis weight profile can be attained in a different type of headbox design, referred to as dilution headbox, in which the basis weight profile of the paper web is controlled by dilution of the stock fed into the headbox with water. Usually, the water used in the dilution process is white water, and hereby the flow of stock having a high consistency is diluted with a low consistency flow originating from the white water. For example, the headbox can have a series of mixing sections or dilution lines distributed over the width of the headbox. White water is injected into the mixing sections to locally control the stock dilution thereby forming a variable consistency profile leaving the slice opening at a constant volume flow. By adjusting the amount of dilution, i.e. the ratio of high consistency flow to low consistency flow, at a plurality of points of the headbox across the machine, for example in response to a measured basis weight profile from on-line scanners, the basis weight of the web can be controlled in an improved manner and rendered essentially uniform in a cross machine direction. A constant volume flow in a cross machine direction may also have beneficial effects on the fibre orientation profile.
However, in paper machines employing dilution headbox designs, notably when using high performance retention agents, it has been experienced that the paper web produced has varying formation and composition across the width of the web. Notably, it has been found that the paper web has a non-uniform ash content cross profile, thereby producing paper out of specification. In some cases the ash content has been much lower in the lateral areas than in the middle area of the web.